1. Field of the Invention
The present invention relates to an exchange system and, more particularly, to controlling the termination process for incoming calls to an exchange system from a direct-in line.
2. Description of the Related Art
A call terminated at a private branch exchange (PBX) from a public telephone network is generally connected to the attendant console and is then extended to an internal station or a private line (WATS/Tie Lines and so on). Meanwhile, an office exchange connected to the public network is connected, in some cases, to the private branch exchange through direct-in lines in place of ordinary lines. An incoming call from the direct-in line can be directly connected to the internal station, private lines for connecting other PBXs or external lines for transmitting calls to a central office.
The services for processing such incoming calls are generally referred to as DISA (Direct Inward System Access) service or DID (Direct Inward Dialing) service. Hereinafter, a call to receive such services will be referred to as a DID call.
The PBX service handles incoming calls in accordance with a class-of-service by assigning a class-of-service to DID trunk circuits provided as an interface for the direct-in line. Therefore, the PBX services provided for DID calls are preassigned for DID calls in accordance with the class-of-service assigned to the DID trunk circuit.
State of the art PBX circuits determine the destination of a call by receiving a dial signal number from the telephone office. In recent years, with development of the Integrated Service Digital Network (ISDN), when a call is terminated by the ISDN line from the ISDN terminal connected to the telephone office as it is received in the form of a message, the destination of the call is determined depending on termination address information inserted in the message. For the termination of calls from the ISDN line, the termination numbers, for example, can be assigned to the call set-up or related message.
FIG. 1 shows connections between a public telephone network and a PBX as background for the present invention. The PBX comprises a switching network 100, a controller 101 for controlling the switching network 100, a memory 102, telephone sets (TEL) 106 connected to the switching network 100 through a line circuit (LC) 104, an attendant console (AT) 107 connected to the switching network through an interface circuit (IF) 105, and trunks (TK) 103 connected to the public network (office line) or private lines (WATS/Tie Line and so on) connected to other PBXs. The other PBXs and the central office exchange (CO) basically have structure similar to the PBX and comprise networks 110, 120, controllers 111, 121, memories 112, 122 and trunks 113, 123 as an interface with the public and private lines.
With the structure described above, the Direct Inward Dialing (DID) service has patterns of termination shown in FIG. 2A, FIG. 2B and FIG. 2C. FIG. 2A shows an example in which the lower digits of a line number to which a call is terminated from the office line correspond to a fixed number of an internal station. As an example, if the direct-in line number is "123-4567", the controller 101 receives the lower digits 567 from the central office line through the trunk TK and terminates the call at the internal station identified as the destination by these digits. Termination of this type is possible because the terminal address provided in the direct-in line number.
FIG. 2B shows a pattern of termination in which when the lower digits 567 of the call terminated from the direct-in line are received, the digits are converted variably to the extension 200 using a conversion table stored in memory 102. Thus, the converted signal is terminated to the extension telephone set as its destination.
FIG. 2C shows an example of termination to an extension in accordance with an additional code. For example, by receiving an additional code number 300, unlike the above two examples where the code sent from the central office exchange is used or converted, the extension can be identified using additional digits sent with the termination address information. The present invention refers particularly to the direct-in line dialing (DID) function for determining the destination of termination by receiving such an additional code.
FIG. 3 shows a diagram for explaining a structure of a PBX system having the direct-in line function of the prior art. The following explanation is based on an example of the ISDN system.
The reference numeral 151 designates a controller of private branch exchange; 152 designates a message analyzing part for analyzing messages such as a call set up message or a PBX release request message; 153 designates a dial digits deciding part for deciding dial digits in the message; 154 designates a call controller for controlling call set up; 155 designates a PBX network; 156, 157 designate trunks, 158, 159 designate ISDN office lines; 160 designates an exchange network including an ISDN exchange; and 161, 162 designate ISDN terminal equipment.
The switching network 160 accommodates the other PBXs not illustrated (such as the other PBX shown in FIG. 1) and ISDN terminal units and the extension lines of PBX network 155 accommodate various terminal units. For instance, the termination address in the call set-up message can be converted to the dial-in number for issuing a call to the ISDN terminal unit 162 from the ISDN terminal unit 161 using the dial-in function of the PBX system.
In FIG. 3, a call is terminated to a trunk 156 from the exchange network 160, for example, through the ISDN office line 158 and the call set-up message is transferred to the controller 151. The controller 151 of the PBX system analyzes the type of message received in the message analyzing part 152 and transfers the termination subaddress (ex: called party subaddress) to the dial digits deciding part 153 for deciding the dial-in number, etc. The result is sent to the call controller part 154 which calls the ISDN terminal unit 162 and controls the setting of a path to the ISDN terminal unit 161 on the call originating side, upon response from the ISDN terminal unit 162.
The PBX system described above is capable of providing a variety of services in addition to the dial-in service discussed above. For example, in the PBX system the service for originating a call to the office line from the external ISDN terminal unit 161 through the PBX network 155 is available. In this case charging for origination of the call to the office line is carried out for the PBX and charging for use of office line extended up to the PBX is carried out for the external ISDN terminal unit 161. Therefore, charges for long-distance communication by telephone can be shared among companies having PBXs installed.
FIG. 4 shows a control flowchart of the prior art. In step 221, the call set-up message is received from the switching network 160. In step 222, the existence of a termination subaddress in the received call set-up message is decided. If the termination subaddress is not provided with the call (i.e., added thereto), then in step 223 the call is terminated to the attendant console or the termination of call is rejected. If the termination subaddress is provided, then in step 224 the termination subaddress is analyzed in accordance with the class-of-service restriction previously assigned to the incoming trunk to determine an internal station number.
When calls are terminated through the same ISDN line in the PBX of the prior art, the class-of-service (class restriction) available are forced to become equal thus losing the relationship to the type of external ISDN terminal unit. The system assigns class-of-service numbers to incoming trunks which are employed to provide the services such as origination of call to the office line. Accordingly, when a call is terminated to an incoming trunk having only the class-of-service allowing termination to the internal station (extension), then only the service of termination of call to the extension is offered. Therefore, as described above, the termination subaddress is analyzed depending on the class-of-service preassigned to a incoming trunk.
In step 225, it is determined whether a service, such as origination of call to the office line, is allowed or not. If the service is not allowed, then in step 226 the call is terminated to the attendant console or rejected. If, on the other hand, the service is allowed, then in step 227 the service such as origination of call to the office line designated by termination subaddress is offered.
As previously explained, since the PBX system having the dial-in service according to the prior art sets up the class-of-service available corresponding to incoming trunks, the number of trunks which can offer a particular service such as origination of call to the office line may be restricted. Thus, a problem of the prior art is that the application efficiency of incoming trunks is low.
Moreover, in a case where a call is terminated to the incoming trunk which allows the service such as origination of call to the office line from the ISDN line, every user can enjoy such service. Thus, the significance of providing service restrictions is lost. It is accordingly a problem of the prior art that the class-of-service cannot easily be changed.